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THE EVOLUTION OF CODA

THE EVOLUTION OF CODA. M. Satyanarayanan Carnegie-Mellon University. Paper overview. Reviews the multiple contributions of Coda: Optimistic replication Trickle reintegration to support weakly connected workstations Isolation-only transactions Operation shipping

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THE EVOLUTION OF CODA

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  1. THE EVOLUTION OF CODA M. Satyanarayanan Carnegie-Mellon University

  2. Paper overview • Reviews the multiple contributions of Coda: • Optimistic replication • Trickle reintegration to support weakly connected workstations • Isolation-only transactions • Operation shipping • Ends with a few lessons learned

  3. MOTIVATION FOR CODA • AFS was found to be vulnerable toserver and network failures • Not that different from NFS • Limits scalability of AFS • Coda addresses these problems through optimistic replication

  4. SERVER REPLICATION (I) • Optimistic replication control protocols allow access in disconnected mode • Tolerate temporary inconsistencies • Promise to detect them later • Provide much higher data availability • Optimistic replication control requires a reliable tool for detecting inconsistencies among replicas • Better than LOCUS tool

  5. SERVER REPLICATION (II) • Unit of replication is volume (subtree of files) • Set of servers containing replicas of a volume isvolume storage group (VSG) • Currently accessible subset of VSG isaccessible volume storage group (AVSG) • Tracked by cache manager of client (Venus):

  6. Read protocol • Read-one-data, read-all-status, write-all • Each client • Has a preferred server (VS) • Still checks with other servers to find which one has the latest version of a file • Reads are aborted if a conflict is detected • Otherwise a callback is established with all servers in AVSG

  7. Update protocol • When a file is closed after modification, updated file is transferred in parallel to all members of the AVSG • Directory updates are also written through to all members of AVSG • Coda checks for replica divergence before and after each update • Update protocol is non-blocking

  8. Consistency model • Client keeps track of subset s of servers it was able to connect the last time it tried • Updates s at least every tau seconds • At open time, client checks it has the most recent copy of file among all servers in s • Guarantee weakened by use of callbacks • Cached copy can be up to tau minutes behind the server copy

  9. Fault-tolerance • Correctness of update protocol requires atomicity and permanence of metadata updates • Used first Camelot transaction management system: • Too slow and Mach-specific • Coda uses instead its own recoverable virtual memory (RVM) • Implemented as a library

  10. DISCONNECTED OPERATION (I) • Started as tool allowing a client isolated by a network failure to continue to operate • Made possible thanks to • Optimistic philosophy • File hoarding in client cache • Gained importance with arrival of portable computers • Resulted in voluntary disconnections

  11. DISCONNECTED OPERATION (II) • File Hoarding: • Coda allows user to specify which files should always remain cached on her workstation and to assign priorities to these files • When workstation gets reconnected, Coda initiates a reintegration process • Changes are propagated and inconsistencies detected

  12. DISCONNECTED OPERATION (III) • Disconnected operation mode complements but does not replace server replication • Cached replicas are only available when client workstation is turned on • Make server replicas primary replicas and cached replicas secondary replicas

  13. Implementation (I) • Three states: • Hoarding:Normal operation mode • Emulating:Disconnected operation mode • Reintegrating:Propagates changes and detects inconsistencies

  14. Implementation (II) Hoarding Emulating Recovering

  15. Implementation (III) • Coda maintains a per-client hoard database (HDB) specifying files to be cached on client workstation • Client can modify HDB and even set up hoard profiles

  16. Implementation (IV) • In disconnected mode: • Attempts to access files that are not in the client caches appear as failures to application • All changes are written in a persistent log,the client modification log (CML) • Venus removes from log all obsolete entries like those pertaining to files that have been deleted

  17. CONFLICT RESOLUTION • Coda provides automatic resolution of simple directory update conflicts • Other conflicts are to be resolved manually by the user

  18. Objectives • No updates should ever be lost without explicit user approval: conflicts must be detected • The common case of no conflict should be fast • Conflicts are ultimately an application-specific concept: think of updates to a schedule • The buck stops with the user: automatic conflict resolution cannot solve all problems

  19. Approaches to conflict resolution • Syntactic approach: • Uses version information • Fast and efficient • Weak in their ability to resolve conflict • Semantic approach: • Slower but more powerful

  20. Coda solution • Coda uses • Syntactic approach to detect absence of conflicts • Semantic approach to resolve possible conflicts

  21. Directory conflict resolution • Always automatic • Uses a log-based • Two cases to consider • After disconnected operation • Across conflicting replicas

  22. After disconnected operation • Each server tries to apply the client modification log (CML) send by the client during reintegration • If this attempt fails, client directory is marked in conflict.

  23. Across divergent replicas • Each server replicas of a volume has a resolution log containing entire list of directory operations • In reality, it is frequently truncated • Remains almost empty when there are no failures • Recovery protocol locks the replicas merges the logs and distributes the merged logs.

  24. Other solutions • Must keep track of partial deletes: • If one of the two replicas has a directory A, does it correspond to a file • recently created, or • recently deleted. • Must keep ghost entries for directory entries that were recently removed • Hard to know when these entries can be purged

  25. Application-SpecificFile Resolution • Entirely done at client

  26. Conflict representation • Coda displays read-only versions of inconsistent objects

  27. Frequency of conflicts • Probability of two different users modifying the same object less than a day apart is less than 0.0075

  28. WEAKLY CONNECTED OPERATIONS • Broad principles • Do not punish strongly connected clients • Do not make life worse when disconnected • Do it in the background if you can

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